Picking up on a weird increase in trades for "earbuds", a rare item in the game, his investigations led him to discover a closed Steam group that was involved in highly suspicious item trading. The report is a lengthy and surprisingly fascinating read, but for the time-starved, let's cut to his tl;dr hypothesis:

tl;dr A group of Russians bought >5,000 Keys from Store today using illegitimate credit cards and bought few hundred Buds from TF2 Outpost at an ridiculous price of 28-30 Keys (The average yesterday was 25.5 Keys). They have been doing this for quite a long time and their activity today is alarming.

One motive for this could be the hilariously bizarre (if unlikely) possibility that Russian gangsters are using Team Fortress 2 to launder dirty money. Another, more likely outcome is that a band of crims have got hold of some stolen credit cards, and are spending the cash on rare TF2 items they can re-sell.